Additional index words. metolachlor, adsorption, media, ornamental, Freundlich isotherms Abstract. Adsorption, mobility, and filtration ability of organic media toward metolachlor were evaluated in a series of laboratory experiments. Experimental variables included media type, metolachlor concentration, and equilibration time. Adsorption isotherms were determined by applying the log form of the Freundlich equation. Mobility was evaluated using glass columns filled with media, which were then surface spiked with metolachlor and then leached daily for 10 consecutive days. Peat, pine bark, combinations of these two media and a mixture of pine bark and sand adsorbed >90% of the 14 C metolachlor. Freundlich sorption coefficients were 10.9, 18.2, 13.4, 14.2, and 11.0 for pine bark, peat, 5 pine bark : 1 peat, 3 pine bark : 1 peat, and 5 pine bark : 1 sand, respectively. In a timed exposure experiment using bark, minimum metolachlor adsorption (57%) was at 90 seconds and maximum adsorption (82%) required at least 1440 minutes. In column leaching studies, data for all media indicate that metolachlor is relatively immobile through these substrates. An initial pulse of metolachlor ( 0.5 μg·liter -1 for each medium) in the metolachlor recovered. Filtration efficiency of commercially formulated metolachlor from water passed through different lengths of pine bark filled filters was 0%, 17%, 20%, 22%, 23%, and 29% for filters 4, 20, 12, 8, 16, and 24 cm in length, respectively. These results support the contention that such filtration would be effective provided the residence time of water within the filter was sufficient for adsorption of the contaminant by the media to occur. Nursery and landscape crops generally require frequent irriga- tion and pesticide application. Irrigation runoff from landscape plant production areas and landscape developments has been suggested as a source of pesticide contamination for local water supplies or surrounding bodies of water (Keese et al., 1992). Holding ponds can be used to reduce the amount of pesticide contaminated water from a container production area. However, where containment ponds are unavailable or not practical, other means of remediation of pesticide contaminated water may be required. Filtration of pesticide contaminated water using con- tainer production organic materials, such as pine bark, or peat, could be achieved using these materials as an adsorbate. Adsorption of oxadiazon and oryzalin in both organic media and a mineral soil was evaluated in previous research (Wehtje et al., 1993, 1994). Averaged across all media and a mineral soil, with herbicide concentrations that ranged from 0.1 to 100 mg·liter -1 (dry-weight basis), the proportion of oxadiazon and oryzalin adsorbed was 98% and 94%, respectively. Column leaching stud- ies revealed that mobility was likewise limited. Neither herbicide was detected in either the leachate or deeper than 3 cm below the surface. It has been concluded that the detection of either oxadiazon or oryzalin in runoff water results from the material not reaching the media due to method of application (Gilliam et al., 1992). The generally high adsorptivity of horticultural media to herbicides has been observed by others (Moles and Whitcomb, 1976; Wilson et al., 1994). Among the herbicides registered for use in nursery crop production, oryzalin and oxadiazon are not likely suspects for leaching. Neither material has been reported to be subject to leaching when used in field situations (Ahrens, 1994), and both are relatively insoluble (Camper et al., 1994). The water solubilities of oxadiazon and oryzalin are 0.1 and 2.6 mg·liter -1 , respectively more...